The invention relates to a device for coupling electromagnetic radiation, comprising at least a first strip-shaped radiation guide which on at least one end bifurcates into at least two further strip-shaped radiation guide, which radiation guides are all bounded by a material having a smaller refractive index for the said radiation than the radiation guides.
Such coupling devices are suitable inter alia for use as a wavelength multiplexing and demutiplexing device in optical glass fibre communication systems and as an optical coupling device in an optical glass fibre gyroscope. More in general these coupling devices which in a certain respect may be considered as the solid-state analogon of a semi-permeable mirror may constitute an important part of opto electronic devices which are referred by to the term "integrated optics".
A coupling device of the type described is known from European Patent Application published under no. EP 0152991. This Application describes an integrated wavelength multiplexing and demultiplexing device consisting of a strip-shaped radiation guide which is provided on a substrate and which at each end bifurcates into two further strip-shaped radiation guides. All radiation guides are bounded by a material having a smaller refractive index for the radiation used than the radiation guides. This device will hereinafter be referred to by the term "horizontal double Y-shaped structure". This structure may be considered to be built up of two single horizontal Y-shaped structures. In the above-mentioned Patent Application glass, lithium-niobate and gallium arsenide are mentioned as materials from which the devide may be formed. In the case in which the substrate consists, for example, of lithium niobate, a certain number of titanium atoms per cm.sup.3 may locally be provided so that the refractive index at that area becomes larger. In this manner radiation guides can be made. For use of the device as described in the said Patent Application the width of the strip-shaped radiation guides should satisfy certain requirements which will not be further discussed here.
A disadvantage of the known device is that it occupies a comparatively large part of the substrate surface. In fact, in case, for example, of a horizontal Y-shaped structure, the surface area required herefor as a result of the necessarily gradual bifurcation of the two further radiation guides is larger than the overall area of the radiation guides, in this example three. However, the following effect is even more important: it must be possible to connect active or passive optoelectronic components to the free ends of the two further radiation guides. These often have dimensions of a few hundreds of microns. Therefore the free ends of the further radiation guides must be situated at a distance from each other of, for example, a few hundred--say 200--.mu.m. Due to the necessarily gradual bifurcation mentioned hereinbefore --which means, for example, that the tangent of the angle between the two further radiation guides must be smaller than 1/60--the necessary length of the further radiation guides becomes very large. For the given numbers the length must be larger than 60.times.200 .mu.m (=12 mm).
A further disadvantage associated with the disadvantage mentioned hereinbefore is, that each active or passive optoelectronic component which is integrated with the device and is coupled to one of the three radiation guides of the Y-shaped structures occupies a part of the surface.
Finally a technological disadvantage must be mentioned, that is to say a disadvantage which is bound to the manufacturing method. It will be obvious that due to the fact that it is very desirable that both the thickness and the width of the strip-shaped radiation guides should by very uniform, stronger requirements must be imposed on these processes during the manufacture of a structure having several radiation guides by means of growing and diffusion or etching when the surface of the structure is comparatively large.
One of the objects of the invention is to avoid the said disadvantages to a considerable extent and notably to make the surface necessary for realising the device including the surface which is necessary for active or passive opto-electronic components connected to the device and integrated therewith as small as possible.